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Network Communication Technology Chapter 20 Asynchronous Transfer Mode (ATM)

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1 Network Communication Technology Chapter 20 Asynchronous Transfer Mode (ATM)


3 ATM Cell Fixed size cell (53 Bytes) Telephony Standard – Intended as a universal, shared bandwidth service Typically 155 Mbit/sec (OC-3, STS-3) user interface with low latency Figure 20.2

4 ATM Connections Virtual Path Identifier (VPI) –High-level route identifier Virtual Channel Identifier (VCI) –Sub-route identifier Connections –PVC Permanent Virtual Connection –SVC (normal) Switched Virtual Connection Figure 20.3

5 A Railroad Model Figure 20.4 VCI 14VCI 10 VCI 11 VCI 15 VPI=2 VPI=1 VCI=3VCI=6 VCI=9VCI=2 ATM Switch VPI=4 VPI=3 Train Station or cell VCI=3

6 ATM Switch Fast –Simple HW –Fixed cell size Flexible BW sharing –Small cells –Low latency Figure 20.5 Input Output Port 1 VPI/VCI 1/26 1/45 Port 2 3 VPI/VCI 2/45 3/39 Table 20.1: Routing Table

7 ATM Switch Architecture Figure 20.6

8 ATM Connection Setup Figure 20.7

9 ATM Cell Formats User Network Interface (UNI) –Generic Flow Control: Not currently in use –VPI/VCI: Identify the next destination of the cell (which switch) –Payload Type: Data/control, congestion, terminating –Congestion Loss Priority: High/low, determines QoS –Header Error Control: Weak CRC on header only Network-to-Network Interface (NNI) Figure 20.9: UNI Cell Header Figure 20.10: NNI Cell Header

10 ATM Layered Architecture Figure 20.11

11 ATM Adaptation Layer 1 (AAL1) Constant bit rate traffic i.e. 64 Kb Telephony Figure 20.12

12 ATM Adaptation Layer 2 (AAL2) Variable bit rate traffic i.e. Compressed audio Figure Information CS Sub-Layer HeaderPayloadTrailer bytes SAR Sub-layer CSISCIT 1 bit 3 bits 4 bits 6 bit 10 bits PayloadLIFEC Cell headerPayload ATM Layer Cell HeaderPayload

13 ATM Adaptation Layer 3/4 (AAL3/4) Variable Frame length Error control Variable delay Figure Information < 65K Bytes PadBEtagALTypeBEtagBA CS Header Length CS Trailer CS-PDU Bytes SAR HeaderCS Header40 bytes PayloadSAR Trailer bytes SAR Header44 bytes PayloadSAR Trailer SAR-PDU STSNMID LIFCS bits 6 10 bits

14 ATM Adaptation Layer 5 (AAL5) Efficient LAN emulation Control info in the last cell Figure CS-PDU CS trailer SAR-PDU ATM Layer <65535 Bytes Bytes PADUUCPILengthFCS Information from upper layer 48 Bytes Payload 48 Bytes Payload 40 Bytes Payload 8 Bytes CS Trailer Cell Header 48 bytes Payload 48 Bytes Payload Cell Header 48 Bytes Payload

15 ATM vs. Gigabit Ethernet ATM –Supports real-time –Offers QoS support –Good as a WAN Gigabit Ethernet –QoS add-on (802.1Q) supports priority delivery –Low cost –Replaced ATM in LAN backbones, also used for short haul WAN connectivity

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